Videos are generally projected as sequences of images on a video screen by a video projection system composed of a single video projector or multiple video projectors generating adjacent, partially overlapping sub-images. Projected images can be of a standard definition or of a high definition (HD), offering high image quality.
When the video projection system used comprises multiple video projectors, each single video projector generates a sub-image with a given definition and a size determined by the video projector lens focal length, the size of the video projector's light modulation device (e.g. an LCD panel), and the distance between the video projector and the video screen. Increasing the projection distance yields a larger but darker projected image (the brightness decreasing with the square of the distance).
Covering efficiently, in terms of brightness, a very large projection screen with the required definition usually requires aggregating several sub-images in a manner that they cover adjacent, partially overlapping zones of the full screen area. In the overlapping zones, blending ensures a smooth transition between sub-images projected by different video projectors in a manner that is tolerant of small displacements introduced, for example, by vibrations or thermal expansion. Blending is a well-known technique that consists in continuously decreasing the brightness of sub-images generated by a video projector towards the border of the projection zone covered by that video projector and complementarily increasing the sub-image brightness of sub-images generated by adjacent video projectors in a manner to obtain uniform brightness after superposition.
Video projectors are commonly equipped with zoom lenses (i.e. lenses with variable focal length) to provide users with freedom in installing a video projector according to given spatial constraints and in choosing the distance between the video projector and the projection screen, as well as letting them flexibly adjust the projected image size versus brightness during a projection session. Typical zoom ranges are from 1.2:1 to 1.7:1. In higher-end projectors the optical zoom is usually motorized and can be actuated through remote control.
The partitioning and blending of sub-images to be projected by a video projection system comprising multiple video projectors have to be carefully adjusted to give the user the impression of a single image throughout the whole projection screen area. To that end, each video projector of a video projection system comprising multiple video projectors is set up so as to control overlapping and blending of areas of the projected sub-images.
This is generally obtained through a calibration process that is typically carried out during system installation or power-up. It often requires user interactions such as taking digital calibration pictures of the screen. Keystone correction and digital rescaling can be performed according to the method disclosed in U.S. 2009/0201431.
However, it has been observed that actuating the optical zooms of video projectors belonging to such a video projection system changes the relative width of the overlapped zones, and hence destroys the calibrated image partitioning and the impression of continuity.
Through optically zooming the lenses of the projectors (e.g. shortening the focal lengths to increase the image size on the screen), the overlapping/blending areas become larger not only in absolute terms but also relatively to the displayed image. Consequently, formerly distinct parts of the image become overlapped and formerly superposed (blended) parts appear duplicated at distinct screen positions. Moreover, the aspect ratio of the display rectangle used for projecting the video images is altered.
Accordingly, there is a need to make the video projection system comprising multiple video projectors behave in a manner analogous to what the user is likely to expect due to her/his experience with single-projector systems offering optical zooming facility.
In particular, there is a need to control image aspect ratio, typically the original 16:9 aspect ratio and for automatically readjusting the partitioned sub-images projected by individual projectors to take into account the larger overlapping and blending areas.
Furthermore, it is desirable to limit the necessary calibration step(s) to the initial system set-up phase (installation or power-up) and to keep it as simple as possible from the user's point of view (once the initial calibration has been done, optical zooming should work in without defects as it does with a single projector).